Capillary electrometer apparatus



Nov. 25, 1947'. w. H. BUSSEY CAPILLARY ELECTROMETER APPARATUS e Filed Jan. 22, 1945 Patented Nov. 25, 1947 UNITED STATES PATENT OFFICE CAPILLARY ELECTROMETER APPARATUS William H. Bussey, Chicago, Ill., assignor by mesne assignments, to Oliver W. Storey, Wheaton, 111., as trustee for the partnership of 0. W. Storey & Associates, Chicago, Ill.

Application January 22, 1945, Serial No. 573,968

17 Claims. 1

This invention relates to an electrical apparatus and particularly to a reversible transducer capable of transforming electrical energy into mechanical energy or vice versa.

As a general rule, transducers are divided broadly into two types, electromagnetic and electrostatic. In the electromagnetic type of transducer, conversion of electrical energy into mechanical energy is through the intermediary of a magnetic field and the mechanical interaction between a magnetic field and magnetic materials or current-carrying members. Such types, whether of the electromagnetic or electrodynamic are fundamentally similar. Devices of this type are characterized by relatively massive structures and have vibrating systems operating in air gaps. The complexity of transducer action involving a change of electrical energy to magnetic energy and interaction of magnetic fields and magnetic materials or current carrying members makes faithful transducer action difficult, because of the varying laws controlling the energy transformations.

Electrostatic transducers in their simplest form comprise a condenser with at least one movable plate. Such a plate will move either in response to a potential change or will generate a potential as a result of movement. Devices of this character are of comparatively low efficiency. The variation of capacitance must be large enough so that its effect on such devices as an amplifier are sensible. Hence comparatively large plate areas or small spacing, or both, are necessary.

Such requirements are a serious deterrent for any acoustic device. Thus large plate areas necessarily result in large motional impedance. Close spacing between two extensive areas also involves comparatively high damping. The combination of large plate areas and close spacing results in low efliciency and seriously restricts the use thereof.

I have discovered that a simple, sensitive, and dependable transducer may be made by utilizing a phenomenon involved in the operation of a capillary electrometer. This type of electrometer involves the interaction of two immiscible liquid surfaces such as mercury and an electrolyte, as dilute sulphuric acid, in a restricted space such as is present in an insulating capillary tube.

While the exact theory of operation of a device of this character is not entirely clear, it would appear as if the interface at the meniscuses of the two liquids acts like a condenser having a relatively high capacitance. The interface will change upon the application of a potential difference to the mercury and sulphuric acid, or if changed, will generate an electromotive force.

The transformation involves potential difference and meniscus travel as the two interrelated factors.

A capillary electrometer includes not only an interface but requires means for applying potential to the two liquids meeting at the interface. Thus in a mercury-electrolyte interface, the liquid mercury may be continued at a distance from the interface to a region where a solid electrode may be disposed therein. Such an electrode may be of any metal or material inert to mercury. Thus platinum or tungsten Or any other suitable metal may be used.

With an electrolyte, such as dilute sulphuric acid, it is undesirable to dispose a solid electrode directly therein. The behavior of the electrometer under such conditions is generally unsatisfactory, and may be due to the presence of a contact potential biasing the interface to an undesirable portion of its characteristic curve. In any event, it is customary to insert between a, solid electrode and the electrolyte some liquid material such as mercury or a so-called calomel electrode.

The interposition of mercury between electrolyte and solid electrode creates a second interface. This second interface may, if desired, be disposed in a region having a much larger cross section than the capillary region in which the desired first interface operates. Thus, the second interface may move with the electrometer interface, but the movement is so small as to be negligible. It is preferred, however, to have the second interface in the capillary to provide two cascaded interfaces.

In physical construction, an electrometer re quires a capillary length in which the electrometer interface may move. Within this capillary length, there may be one or more electrometer interfaces. As a rule, each end of the capillary communicates with a larger region and means, such as a compressible gas space, is provided for permitting liquid movement.

I have discovered that, by combining a Bour don tube with a capillary electrometer, efiective and reversible transducer action'may be obtained. As is well known, a Bourdon tube consists of an elastic tube, closed at one end and curved in one of any number of possible shapes. The tube may be inherently elastic and provides its own restoring force or may have a separate spring for performing this function. A Bourdon tube of necessity has a non-circular cross section at the part thereof that is curved in one of any number of possible shapes. As used hereinafter, the expression Bourdon tube is limited to a structure as defined above.

When this tube is subjected to an internal pressure, it tends to vary its curvature. Thus a tube, whose outer surface is at atmospheric pressure, and whose cross section is non-circular and which is bent from a straight line, will tend to straighten out to a circular cylinder when subjected to an internal pressure above atmos phere. The use of the expression Bourdon tube herein contemplates not only variations in mechanical structure; 1. e. different curvature of tubes, or having a tube of soft material with a separate spring; but also contemplates its functional reversal; i. e. moving the tube in response to pressure changes or generating pressure changes by moving the tube. By coupling a Bourdon tube section to one end of a capillary in which at least one electrometer interface is disposed and providing compliant means at the other end of said capillary, energy conversion is possible with electrometer operation.

It is preferred to endow the Bourdon tube system with considerable stiffness and utilize a substantially incompressible medium for transmitting pressures to the Bourdon tube. Thus, mercury from the capillary may be extended to the Bourdon tube section to fill the same substantially completely. The other end of the capillary is preferably in communication with a compressible medium such as gas. It is preferred to have some compressible medium such as gas provide a means for permitting liquid movement and also take up variations in volume of liquids due to temperature changes.

In practice, the portions of the electrometer coupled to the ends of the capillary generally will both contain mercury. The Bourdon tube portion or the system with its incompressible liquid may be considered as that portion of the system in which the volume changes. The remaining portion of the system has compliance and may contain a compressible gas.

In one specific form, the invention provides a Bourdon tube section formed in a generally U-shape and sufiiciently elastic so that pressure variations incident to normal electrometer operation will affect the Bourdon tube portion.

It is well known that the action of an electrometer is influenced by the purity of the liquids therein. Under certain conditions, it may be necessary for some slight aging to occur in the case of a mercury-sulphuric acid couple. However, since capillary electrometers have been well known and investigated, it is unnecessary to go into detail on such matters.

I The capillary type of electrometer having mercury and electrolyte liquids must be used below a threshold potential of about one-half a volt for each interface of equal area for reasons given hereinafter. If the interfaces are considered as condensers, for purposes of analysis, the reason for approximate equality of area becomes clear. Condensers in series divide the potentials inversely according to capacitance. Excessive potentials result in electrolysis and generation of gas and other products. This tends to impair or 4 destroy the action of the electrometer interface.

In many instances, it is desirable to utilize potentials in excess of one-half a volt and to this end, it may be desirable to provide a series of cascaded interfaces. Thus, the capillary portion may have minute quantities of mercury and electrolyte built up to form a series of interfaces. Because of relatively high electrical resistance, it is preferred to keep the quantity of electrolyte to a minimum. The end liquids may then be treated exactly as in a single interface electrometer. In electrometers of this character, the movement of the various interfaces will be cumulative and in a similar direction. Thus, a cascaded series of interfaces may provide a transducer in which substantial potentials may be either generated or utilized.

It is understood that whatever liquids may be used for capillary electrometers may also be used in the transducer. Thus, instead of dilute sulphuric acid, other liquid electrolytes such as aqueous solutions of the hydroxides of sodium or potassium and the alkali metal halides, and other salts both organic and inorganic have been used.

Referring to the drawing,

Figure 1 is a sectional elevation of a structure embodying the invention.

Figure 2 is a section on 22 of Figure 1.

Figure 3 is a section on 3-3 of Figure 1.

Figure 4 is a detail of the capillary showing a series of cascaded liquid columns.

In Figure 1, a tubular system is shown wherein a pair of tubular members I and 2 are connected together by at least one capillary portion 3. Capillary 3 preferably has a fine bore 4. This bore, in practice, may range from around twenty v to around fifty microns and may go higher. It is ally U-shaped flattened tube.

possible to use bores down to about ten microns. Below this value, undesirable secondary effects are apt to occur. However, under certain conditions, large bores may be used. Thus a bore of one millimeter has been used successfully. Larger bores may be used under certain conditions where high frequency response is not important and where a low impedance is not objectionable. It is preferred to have capillary 3 provided with relatively heavy walls for mechanical strength.

The actual length of capillary 3 may vary. In practice, however, lengths of the order of between one-eighth and three-quarters of an inch may be used. Between capillary 3 and tube members l and 2 may be tapered portions 5 and B.

Member 2 continues on to Bourdon tube portion 1 which, in this instance, comprises a gener- Bourdon tube portion 1 continues to terminal portion 8 within which an electrode 9 may be sealed. It is preferred to have the Bourdon tube portion together with adjacent tube portion 2 and terminal 8 as short as possible in order to reduce the amount of mercury required for operation. However, in order to obtain a leverage action, it is preferred to continue the structure in the form of extension l0.

Within Bourdon tube portion 7 and tube portion 2, a quantity of mercury H is disposed. It is preferred to have mercury I! fill the entire space between capillary 3 and sealed end 8. In practice, some microscopically small quantities of air may be present.

Mercury ll extends up into capillary bore 4 within which there may be disposed a quantity of electrolyte l2. As electrolyte, a dilute soluion of sulphuric acid has been successfully used. It is preferred to keep the quantity of electrolyte as small as possible, since it has a higher electrical resistance than does the mercury. However, if desired, electrolyte I2 may extend up capillary bore 4 and up into taper portion 5 or even tube portion l. Beyond electrolyte l2, mercury l3 may be disposed, and this preferably extends well up into tubular portion l. Above mercury l3, gas space 4 is formed. This gas space may be filled with either air or some inert gas at any desired pressure. It is possible that gas space M may provide some elasticity for the liquid movements of the electrometer. However, by controlling the ratio of elasticity to mass of the entire system, it is possible to set the resonance frequency of the mechanical system at any desired value.

Electrode l5 may be sealed in the free end of tubular portion I, this electrode being sufficiently long to dip into mercury body I3 under all conditions of operation.

Within capillary bore 4, mercury and electrolyte will meet at some predetermined region to form at least one interface Hi.

The entire structure for the tubular system may be formed of glass, quartz, plastic material, or any other insulating material. It is, of course, possible to make tubular portions i and 2 and capillary portion 3 of inelastic material and make Bourdon tube portion 1 of elastic material. In this connection, there may be observed, as has heretofore been pointed out, that Bourdon tube portion 1 may have a separate restoring spring with the tubular portion thereof being formed of flexible material.

It is preferred, however, to make the entire tubular system of glass. As is well known, glass is highly elastic and, within its elastic limits,

exhibits little fatigue.

The structure may be conveniently supported by a pair of collars 20 and 2| of any suitable material. Collars 20 and 2! may have strips 23 and 24 extending therefrom for mounting purposes. The collars may be of metal. In such a case, terminal l5, which in practice would be a fine wire, may be attached to collar 2|, and the collar itself be used as a terminal for one side of the system. A flexible wire may be attached thereto in a manner similar to that of voice coils in loud speakers 01 the like.

A collar 25 may be disposed around extension H3 at any desired point, and this collar may advantageously be of metal and have attached thereto lead electrode 9. Thus, the other terminal of the electrometer could be considered as collar 25 as far as convenient circuit connections are concerned. Collar 25 may have a rigid rod 25 extending between the collar and a diaphgram 2'! suitably mounted for actuation. Diaphragm 21 is merely exemplary and may be any acoustic element or, in fact, may be any load for receiving mechanical movement or vibration, or may be any generator of mechanical movement or vibration.

It is preferred to have some means for limiting the amplitude of vibration of the Bourdon tube po-tion, so that fracture of the glass (if glass is used) will not occur. To this end, collars 20 and 25 may advantageously be disposed substantially in line with each other and have such a space therebetween so that collar 20 will act as a back-stop. A suitable stop may be disposed to limit outward flexure of the tube if that may be necessary.

It is possible to dispose damping material 30 between opposed collars 20 and 25 to control 6 resonance effects. Thus a strip of sponge rubber may be disposed therein. i

An electrometer having one effective interface is limited to comparatively low voltages. Thus, as pointed out above, a mercury-sulphuric acid couple must be used with potentials having peak amplitudes of less than half a volt. In many instances, such a limitation is undesirable. It is possible to modify the electrometer so that larger potentials may be accommodated. Thus, a series of cascaded interfaces may be provided for interspersing short columns of one of the liquids with short columns of the other liquid.

In Figure 4, an enlarged detail of capillary portion 3 of the electrometer is shown. Beginning with meniscus 25 formed by mercury column 26 from the main body of mercury in tube 2, a small quantity of electrolyte, in this case dilute sulphuric acid 28, is disposed on top. Above electrolyte 28 is some mercury 29, then electrolyte 30, mercury 3|, electrolyte 32 and so on, until finally the main body 35 of mercury is reached. Due to the various surface tensions, a system of discrete liquid particles will exist in a capillary in more or less stable form. As is well known, sharp mechanical shock may change the system and result in coalescence or further sub-division.

Under normal conditions where only one capillary interface is present, meniscus 25 would change or move up or down with reference to some neutral position. Where additional meniscuses are provided, as is true in this figure, the change or motion of each meniscus is added to that of the other meniscuses. Thus, with one meniscus, the acid may be moved up or down to accommodate the meniscus travel. With several meniscuses, the various bodies of liquid 28 to 3| inclusive are moved up and down, the resulting movement being transmitted to the main body of acid 35, while the corresponding pressure is transmitted to the main body of mercury 26.

It is clear that, for a series of cascaded interfaces, it may be desirable to provide a capillary section longer than would normally be the case for one or two interfaces.

While the device shown in Figure 1 has been assumed to occupy a vertical position, it is to be understood that the device may be used in any position.

It is understood that the drawing does not show any proportions and that the relative and absolute dimensions of the various portions of the transducer system may be varied within wide limits.

What is claimed is: v

1. In a transducer, a capillary electrometer including two immiscible liquids having at least one interface in the capillary tube portion thereof and a liquid filled Bourdon tube system in pressure communication with said capillary electrometer.

2. In a transducer, a capillary electrometer including mercury and electrolyte having at least one interface in the capillary tube portion thereof and a liquid filled Bourdon tube in pressure communication with said capillary electrometer, said Bourdon tube having a part adapted to move and said interface being susceptible to change, said part movement and meniscus change being concomitant.

3. In a, transducer having a capillary electrometer including two immiscible liquids having at least one interface in the capillary tube portion thereof and a liquid filled Bourdon tube in pressure communication with said capillary electrometer, said Bourdon tube having a part adapted to move and said interface being susceptible to change, the method of translating vibratory energy into electrical energy which comprises vibrating said Bourdon tube to generate volume variations therein in accordance with such vibrations to move liquid in said Bourdon tube, and communicating said liquid movements to the liquids in said electrometer.

4. In a method of vibration generation comprising impressing varying potentials on a capillary electrometer including two immiscible liquids having at least one interface in the capillary tube portion thereof and a Bourdon tube in pressure communication with said capillary electrom eter to cause changes at the liquid interface to generate pressure variations in said electrometer, transmitting said pressure variations to said Bourdon tube system to cause movement there. of and utilizing said tube movements to generate such vibrations.

5. A transducer comprising a tubular system including an intermediate capillary tubular portion joined to form a sealed system, mercury in one portion of said system, electrolyte in another portion of said system, both liquids having at least one common interface in said capillary portion, conductors to said mercury and electrolyte, said tubular system having a part thereof formed of elastic material shaped to form a flattened U -shaped Bourdon tube portion, a load or generator and coupling means between said load or generator and said Bourdon tube portion for transmitting vibratory energy.

6. The structure of claim wherein said Bourdon tube ortion is made of glass.

'7. In a transducer, a capillary electrometer having at least two immiscible liquids, said liquids being broken up and alternately disposed to provide more than two interfaces in said capillary, and a Bourdon tube, said electrometer and tube being in pressure communication with each other and said plurality of interfaces being adapted to accommodate larger potentials than a single interface.

8. An energy-conversion device comprising an insulating container having a capillary passage therein, a quantityof mercury and electrolyte meeting in said capillary to form an interface, means for establishing metallic contacts to said liquids, a sealed container filled with a substantially incompressible fluid coupled to said capillary at one end thereof, a container having means for readily accommodating substantial volume variations incident to interface movement coupled to said capillary at the other end thereof, said first-named container including as a part thereof a Bourdon tube portion,

9. An energy-converting device comprising an insulating member having at least one capillary, mercury and electrolyte forming at least one interface within a capillary channel, mercury columns for conducting current to each liquid forming the interface, solid electrodes contacting said last-named mercury columns, a container coupled to one end of said capillary and filled with substantially incompressible fluid, said container being formed of elastic material having a high restoring force and including as a part thereof a Bourdon tube portion, a second container coupled to the other end of said capillary, said second container having means for readily accommodating substantial volume variations incident to interface movement in said capillary.

10. An energy-converting device comprising a pair of conduits connected together by an insulating capillary conduit to form a sealed system, mercury and an electrolyte forming at least one interface in said capillary, means including mercury for establishing a metallic connection to the liquids at said interface, one of said conduits being filled with a substantially incompressible liquid and including as a part thereof a Bourdon tube portion, the other conduit being filled at least partly with a readily compressible gas.

11. The structure of claim 10 wherein mechane ical linkage means are connected to said Bourdon tube portion for transmitting movements between said Bourdon tube portion and a load or generator.

12. An energy-converting device comprising an insulating capillary having mercury and an electrolyte meeting at least in one interface therein, a glass Bourdon tube section coupled to one end of said capillary, a sealed chamber coupled to the other end of said capillary, means including mercury for establishing electrical contact from the outside of said device to said liquids forming said interface, said Bourdon tube section being filled with an incompressible liquid and said other section having some compressible gas therein.

13. The structure of claim 12 wherein said Bourdon tube portion has a linkage for mechanically transmitting movements between said device and some vibration load or generator.

14. A transducer comprising an elongated tubular system including an intermediate capillary portion, a liquid metal and electrolyte in said system forming one or more active interfaces, conductors to said liquids for impressing a potential across each interface, one end of said tubular system being bent to form a generally U-shaped Bourdon tube section, rigid collars around said tubular system, there being at least one collar on each side of said U-shaped Bourdon tube section, the collar on one side of said Bourdon tube section being adapted to be rigidly maintained while the collar on the other side of said Bourdon tube section is adapted to transmit force between said tubular system and some outside load for movement.

15. The structure of claim 14 wherein conductors have portions that are sealed in said tubular system and are anchored to said collars.

16. In combination, a capillary electrometer including mercury and electrolyte having at least one interface in the capillary tube portion thereof, said capillary tube portion having a capillary bore of the order of from between about ten microns and about fifty microns and a liquid filled Bourdon tube in pressure communication with said electrometer.

17. A transducer of the capillary electrometer type comprising a length of capillary tubing, a Bourdon tube at one end of said capillary tubing, an enlarged tubular part at the other end of the capillary tubing, said enlarged tubular part, capillary tubing and Bourdon tube being of insulating material and integral with the interiors communicating with each other and forming a sealed tubular system, an electrolyt and mercury in said capillary tubing forming at least one electrometer interface, mercury in said Bourdon tube and enlarged tubular part in intimate contact with the interface forming liquids, leads sealed in said structure to establish electrical contact 9 from the exterior with the mercury in said Bourdon tube and enlarged part respectively, said enlarged part having a sensible volume therein filled with gas, said gas being disposed between mercury and the sealed end of said structure.

WILLIAM H. BUSSEY.

REFERENCES CITED The following references are of record in the file of this patent:

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